Is it possible to set up a DD-WRT router as a repeater or client bridge without having access to the gateway you are trying to repeat/bridge?
migrated from serverfault.com Oct 28 '11 at 23:37
No. You can create your own WiFi that's routed to the existing WiFi. You can bridge wired clients to the existing WiFi. But bridging wireless clients will not work.
We tend to think of WiFi as working just like wired Ethernet. But unfortunately, it's really not. It has its own rules and this is one case where it's just different.
The short version of the problem is this: An access point will not accept packets with origin hardware addresses that are not paired with the access point. Since some clients won't be able to reach the existing WiFi network, they won't pair with it, and those access point will reject bridged/repeated packets from them.
What might work, and I stress might, is two bridges back-to-back, connected by wired Ethernet. The specifications say it shouldn't work, but in practice it seems to. Apparently, while the WiFi hardware isn't capable of proxy registration, the DD-WRT software is. Bridging from wired to wireless is done in software in these routers, and DD-WRT is smart enough to proxy register a software-bridged packet. (It knows it has to do the WiFi magic on behalf of its wired clients, since they obviously won't be able to do it themselves.)
For this setup, you connect two wireless routers, LAN-to-LAN. You configure the WiFi on one as a station on the existing WiFi. You configure the WiFi on the other as an access point on a new network. You will probalby need to use a different SSID though, so people will specifically have to choose to connect to your network.
Theoretically, you could use the same SSID and encryption, if any. This would allow clients to automatically connect to the strongest network. You will have two issues. First, you have to make sure your two routers don't connect to each other wirelessly. That obviously won't work. Second, clients will typically show a temporary disruption of service when they switch to and from your access point because the existing network won't know where to find them. MAC learning takes up to a minute. You might get complaints if you 'steal' people who momentarily came into range of your access point and experienced service disruptions as they switch to and from it.
Update: The way back-to-back bridges work is by a form of NAT. The bridge doesn't proxy register its clients but instead re-writes their source hardware addresses.